1. Field of the Invention
This invention relates to improvements in an airbag restraint system, for protecting a passenger from impact during a vehicle collision, of the type in which an airbag is stored within an instrument panel of a motor vehicle.
2. Description of the Prior Art
A variety of airbag restraint systems have been proposed and put into practical use. Of these airbag restraint systems, the type having an airbag stored in its folded state inside an instrument panel is well known. In such a system, when a deceleration during a vehicle collision exceeds a predetermined level, an initiator is ignited to start the operation of an inflator, thereby inflating the airbag from its folded state. The inflated bag protects the vehicle passenger from coming into direct contact with the vehicle's instrument panel and/or front windshield. The airbag restraint system of this type is provided for a vehicle passenger situated on a seat beside a driver. For the driver, another type of an airbag restraint system is provided, in which an airbag is stored in its folded state in a center pad of a steering wheel.
An example of the airbag restraint system for the vehicle passenger is disclosed in Japanese Utility Model Provisional Publication No. 58-23756. Such a conventional airbag restraint system will be discussed with reference to FIG. 1 of the drawings of the present application. In FIG. 1, the airbag restraint system includes an airbag unit 1 assembled in a casing 2. The airbag unit i is fixedly disposed inside an instrument panel 3 and located facing an opening 3a of the instrument panel 3. A lid member 4 is disposed in a position to cover the opening 3a, and formed with low-strength portions 4a near the outer periphery thereof. The low-strength portions 4a are formed thin in thickness as compared with other portions of the lid member 4. An airbag 5 is stored in its folded state within the casing 2 together with an inflator 6. The inflator 6 is arranged to operate to supply a gas to the airbag when a vehicle collision occurs.
When a collision occurs having an impact such that a deceleration of the vehicle exceeds a predetermined level, a collision sensor outputs a signal representing such a high deceleration. In response to such a signal, an initiator disposed in the inflator 6 is ignited, which operates the inflator 6 which supplies the airbag with a high pressure gas. The airbag 5 then pushes the lid member 4, breaking the low-strength portions 4a, and rapidly inflates and develops into a space between the instrument panel 3 and a front windshield 7 in a manner as indicated in phantom M, thereby protecting the passenger on the seat beside the driver's seat from coming into direct contact with the windshield or the instrument panel 3.
However, difficulties have been encountered in the above-discussed conventional airbag restraint system, in which a considerable force pushing the passenger rearward acts from the inflating airbag 5 against the passenger when the vehicle speed immediately before the vehicle collision is lower, since the inflation of the airbag is accomplished only in response to a signal from a collision sensor.
More specifically, when the vehicle collides against an obstacle at a vehicle speed of 50 km/h or higher, a greater forward kinetic energy is developed in the passenger under the action of inertia, so that the kinetic energy is absorbed and dissipated by the inflated airbag 5 thereby softening an impact of the passenger against the instrument panel 3 and/or the front windshield 7. On the other hand, even when a vehicle collision occurs at a vehicle speed of 30 km/h or lower, the airbag 5 inflates in the same manner as when the vehicle encounters a collision at the highest vehicle speed even though the forward kinetic energy of the passenger is much smaller. As a result, the passenger's body during the power speed collision is unavoidably pushed back at a greater acceleration rearward of the vehicle, which is very dangerous for the passenger.